This invention relates to a method of efficiently and economically producing high quality electric welded pipes having precise dimensions.
Generally, electric welded pipes have been manufactured by performing cold rolling of a flat steel sheet into a tube and then by electrically welding the seam of the tube in a cold condition. Since the resistance to deformation of steel strip is large, roll forming in a cold condition is usually carried out gradually using a 9 or 10 stand rolling mill which comprises rough rolling stands such as breakdown roll stands and forming roll stands, and finishing roll stands such as fin-pass roll forming stands. In the prior art, it has been the practice to restrain the edge lines of the steel sheet during rolling by means of side guide rolls and the like. The roll forming process is easily carried out with high precision. The optimum shape for the caliber of a forming roll has been long established as a result of detailed analysis.
However, if roll forming is carried out in a cold condition, as mentioned above, it necessarily requires the use of a large number of roll stands. This inevitably results in a large increase in equipment costs and also requires complicated operations and elaborate maintenance work. Therefore, high manufacturing costs are unavoidable.
Accordingly, it has been tried to produce electric welded tube under hot conditions, i.e., in a temperature range higher than the Ar.sub.3 point, in which the resistance to deformation is very small and working can be performed using small-sized equipment. In the prior art, in the case of "butt welded tube" with a thin wall and a small diameter, roll forming into a tubular shape has been practiced under hot conditions. However, so-called electric-welded steel tube has a thicker wall and a larger diameter and requires a more complicated tube-forming process. It is said that for electric welded steel tube, roll forming under hot conditions has the following disadvantages, and that stable operation and high precision cannot be expected.
(a) Since the resistance to deformation of the starting steel sheet is low in a hot condition, but the edge lines thereof are easily damaged, it is impossible to employ side guide rolls and the like to restrain the edge lines of the sheet. Without such side guide rolls it is rather difficult to carry out roll forming in a stable manner.
(b) Deforming behavior of steel sheet under a hot condition is quite different from that in a cold condition. It is rather difficult in a hot condition to stably produce pipes having not only a precise shape and precise dimensions but also a weld of high quality.
(c) In contrast to cold rolling, the formation of scale during heating and processing is inevitable. Additional work is necessary to remove the scale. Such additional work sometimes disturbs a continuous manufacturing process. Scale remaining on a steel surface also has an adverse effect on the quality of the product.
(d) Hot roll forming is carried out at a point in time after steel sheet has been discharged from a heating furnace and before the sheet is welded. While being roll formed, the starting steel sheet is cooled nonuniformly, resulting in nonuniform temperature distribution around the periphery of the pipe. This temperature nonuniformity also prevents the manufacturing process from being performed smoothly. When the number of roll stands is increased in order to perform more stable roll forming, thermal losses and nonuniformity of the temperature distribution become quite significant, resulting in pipes of degraded quality as well as discontinuity of the process for manufacturing pipes.